A. Field of Invention
This invention generally relates to brominated styrenic polymers suitable for optical film and/or liquid crystal display applications, and processes for their preparation. Some embodiments may have one or more improved mechanical properties.
B. Description of the Related Art
A simplified liquid crystal pixel device is illustrated in FIG. 1. A typical liquid crystal display (LCD) pixel includes a birefringent liquid crystal layer 300 sandwiched between a pair of transparent electrodes 200, 202 such as indium tin oxide glass (ITO). The electrodes 200, 202 include surface treatments that orient the liquid crystal molecules. For instance, in a twisted nematic phase device a first electrode 200 would include unidirectional horizontal grooves 201 aligned with an x-axis, and a second electrode 202 would include vertical grooves 203 aligned with a y-axis. The electrodes are themselves sandwiched between first and second polarizing layers 100, 102 oriented at right angles to each other. Each polarizing layer is aligned with its adjacent electrode so that properly polarized light will pass through both. A light source 400, e.g. a back light, is positioned at one side of the pixel. Typically, the layer of liquid crystal molecules is aligned within the device so that it rotates polarized light passing through it by 90°. Accordingly, light passes through the device from the light source at a back side and is observable at a front side. However, when the liquid crystal layer 300 is energized the liquid crystal molecules reorient in the electric field and thus rotate incident polarized light insufficiently to pass through the second polarizer 102. Therefore, the light is blocked by the second polarizing layer. The net effect is that substantially no light can pass through the pixel while the liquid crystal is energized.
Since the liquid crystal layer is birefringent, the light exiting such a device has two indices of refraction, i.e. a parallel component referred to as “extraordinary” and a perpendicular component referred to as “ordinary.” The magnitude of birefringence Δn is the difference between these two components (see Eq. 1).
The arc through which the image quality is acceptable is regarded as the viewing angle. Importantly, the size of the viewing angle is directly tied to the birefringence of the liquid crystal layer. Specifically, if the parallel and perpendicular components are equal then the viewing angle approaches 180°. However, generally one component is much larger than the other. The larger the difference, the smaller the viewing angle. For example, a liquid crystal layer may have a much higher parallel component. Accordingly, such a device would have a small viewing angle. An optical compensation film brings the parallel and perpendicular components into balance, thereby reducing the magnitude of Δn, and increasing the viewing angle.Δn=no−ne=n⊥−n∥  Eq. 1
Optical compensation films are known in the liquid crystal arts. However, polystyrene is generally regarded as a poor material choice because of its retardation (Γ) instability. As shown in Eq. 2, retardation (Γ) is the product of material thickness (d), and birefringence (Δn). In part this is attributable to its relatively large photoelasticity modulus around a normal operating temperature range. Accordingly, polystyrene's retardation is very sensitive to small stresses. Additionally, the retardation of polystyrene is a strong function of wavelength, and polystyrene has poor heat resistance properties. For these reasons, polystyrene is not typically used for optical compensation films.Γ=Δn·d  Eq.2
Brominated polystyrenes are known in the chemical arts, but have not been applied to the optical compensation film arts because of a variety of persistent problems. For instance, brominated polystyrene products having various degrees of bromination are often incompatible. This incompatibility can be observed by mixing two brominated polystyrenes with different degrees of substitution (DS) in a solvent. For example, mixing two samples of DS=1 (monobrominated) and DS=2 (dibrominated) products in 1,2-dichloroethane would result in a hazy solution, although each are soluble in the same solvent in the absence of the other. The hazy solution is a problem for optical film application since clear films cannot be cast. This problem can occur not only in two different bromination products but also in a single product. A bromination product that is produced by some processes can include molecules with significantly different degrees of substitution, which in turn causes the incompatibility problem and renders the product unsuitable for optical film application.
Furthermore, products that are prepared using higher-temperature processes, longer reaction times, and/or higher catalyst levels can have molecular weights too low to yield films with sufficient integrity due to polymer degradation during the bromination process.
Some embodiments of the present invention provide brominated polystyrenes that overcome one or more of the shortcomings of the prior art, and are suitable for use as optical compensation films.